SAO PAULO/BRASILIA, Oct 23 (Reuters) - Facebook Inc's messaging service WhatsApp has no plans to change its group messaging limits in Brazil, an executive said on Tuesday, resisting calls from the leading presidential candidate to allow easier forwarding of viral messages.

Jair Bolsonaro, the far-right lawmaker forecast to clinch the race on Sunday, vowed last week to "fight" to let users forward text, audio and video messages to hundreds of contacts on a platform that has become a key political battleground.

With few traditional campaign resources and almost no TV ads until this month, Bolsonaro stormed to the front of a crowded presidential race with an outsized social media presence and grassroots organizing via WhatsApp and other online platforms.

He denied accusations from his leftist rival Fernando Haddad last week that he had asked supporters to fund secret bulk messaging campaigns over WhatsApp, which would be a violation of electoral law.

Advocacy groups concerned with an explosion in the number of hoaxes and misleading propaganda texts and videos on the encrypted platform have called on WhatsApp to lower its forwarding limit in the country from 20 recipients to five, as is the case in India.

"We're pretty comfortable with that number 20," Victoria Grand, vice president of policy and communications at WhatsApp, told journalists in Sao Paulo on Tuesday. She called the policy as an "experiment" that "could change over time."

With just five days to go before the vote, she said there was no way that WhatsApp could change those limits before Brazilians choose their next president. She highlighted efforts to shut down accounts engaged in spamming activity, along with a marketing campaign encouraging users to be skeptical of rumors.

Independent fact checkers and social media experts say that has not been enough to stop a flood of falsehoods and conspiracy theories distorting political debate without public oversight.

"It is very worrying. We are walking on ice, because fake news is manufactured on an industrial scale, but the monitoring and fact checking is a slow and time-consuming process," said Thiago Tavares, head of SaferNet Brasil, a non-governmental organization that monitors social media for potential crime.

Tavares, a member of an advisory council to Brazil's top electoral court on social media and the election, is one of many calling for WhatsApp to further restrict forwarded messages that effectively turn the service into a broadcast platform.

Unlike Facebook's open platform, which the company monitors for abusive content, messages circulating in WhatsApp groups of up to 256 use end-to-end encryption, which keeps them hidden even from the platform's administrators.

WhatsApp has more than 120 million users in Brazil, a country of nearly 210 million people, rivaling the reach of Facebook's main platform in one of the company's biggest global markets.

BOLSONARO DECLINES DEBATE

Opinion polls show Bolsonaro has a lead of around 18 percentage points over Haddad, making a last-minute surge for the leftist candidate seem an increasingly distant possibility.

Haddad, running for the Workers Party (PT), has demanded that he and Bolsonaro debate before Sunday anywhere and in any format, saying Brazil's democracy will suffer if voters cannot hear and compare their ideas.

But Bolsonaro said in a radio interview on Tuesday that he would not do that, saying that he was not physically fit enough due to a near-fatal stabbing at a rally last month.

Haddad and his allies have said they are gravely concerned that Bolsonaro, a 63-year-old retired Army captain and seven-term congressman who has openly praised Brazil's 1964-1985 military dictatorship, could fall back into authoritarian ways.

A cadre of high-ranking ex-military brass have supported Bolsonaro and are poised to take powerful roles in government if he wins.

"We are not a threat to democracy. On the contrary, we are the guarantee for liberty and democracy," Bolsonaro said in the radio interview.

He pointed to the political corruption that grew under successive PT governments and involved every major party as evidence that a Haddad government would allow graft to continue. Haddad has said he would fight against corruption.

While his defense of the dictatorship and offensive comments about women and minorities have angered critics, Bolsonaro has not been hit by corruption charges - a selling point with voters weary of economic crisis and graft scandals.

Reporting by Brad Haynes in Sao Paulo and Anthony Boadle in
Brasilia
Additional reporting by Brad Brooks in Sao Paulo and Pedro
Fonseca in Rio de Janeiro
Editing by Frances Kerry and Rosalba O'Brien